mm: softdirty: respect VM_SOFTDIRTY in PTE holes

After a VMA is created with the VM_SOFTDIRTY flag set, /proc/pid/pagemap
should report that the VMA's virtual pages are soft-dirty until
VM_SOFTDIRTY is cleared (i.e., by the next write of "4" to
/proc/pid/clear_refs).  However, pagemap ignores the VM_SOFTDIRTY flag
for virtual addresses that fall in PTE holes (i.e., virtual addresses
that don't have a PMD, PUD, or PGD allocated yet).

To observe this bug, use mmap to create a VMA large enough such that
there's a good chance that the VMA will occupy an unused PMD, then test
the soft-dirty bit on its pages.  In practice, I found that a VMA that
covered a PMD's worth of address space was big enough.

This patch adds the necessary VMA lookup to the PTE hole callback in
/proc/pid/pagemap's page walk and sets soft-dirty according to the VMAs'
VM_SOFTDIRTY flag.
Signed-off-by: Peter Feiner <pfeiner@google.com>
Acked-by: Cyrill Gorcunov <gorcunov@openvz.org>
Cc: Pavel Emelyanov <xemul@parallels.com>
Cc: Hugh Dickins <hughd@google.com>
Acked-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/proc/task_mmu.c

@@ -925,15 +925,30 @@ static int pagemap_pte_hole(unsigned long start, unsigned long end,
 				struct mm_walk *walk)
 {
 	struct pagemapread *pm = walk->private;
-	unsigned long addr;
+	unsigned long addr = start;
 	int err = 0;
+
+	while (addr < end) {
+		struct vm_area_struct *vma = find_vma(walk->mm, addr);
 		pagemap_entry_t pme = make_pme(PM_NOT_PRESENT(pm->v2));
+		unsigned long vm_end;
+
+		if (!vma) {
+			vm_end = end;
+		} else {
+			vm_end = min(end, vma->vm_end);
+			if (vma->vm_flags & VM_SOFTDIRTY)
+				pme.pme |= PM_STATUS2(pm->v2, __PM_SOFT_DIRTY);
+		}
 
-	for (addr = start; addr < end; addr += PAGE_SIZE) {
+		for (; addr < vm_end; addr += PAGE_SIZE) {
 			err = add_to_pagemap(addr, &pme, pm);
 			if (err)
-			break;
+				goto out;
+		}
 	}
+
+out:
 	return err;
 }